Acid catalysis in ionic liquids

The efficacy of Bronsted acid-catalysed esterifications in ILs is dependent upon several different factors including acid strength, biphasie separation 

Alkyl acetates AHq l fatty acid esters AHq l benzoates 80-99% 

The extent of interaction between the anion and cation in the IL has also been shown to affect the esterification reaction. Highly coordinating anions [e.g. trifluoroacetate and tetrafluoroborate), which have strong interactions with the cation, result in lower yields than anions with weaker interactions e.g. triflate and hexafluorophosphate). The more highly coordinating anions are expected to have a larger barrier for the formation of an encounter complex between the cation and the carbojq lic acid reactant, thus raising the barrier for acid catalysis. 

The level of water content in the ILs also influences the yields of the esterification/ For the phosphonium sulfonic BAIL (Table 3.1 Entiy 8) it was demonstrated that 30% (w/w) water resulted in highest yields and percentages of water above or below this value diminished the yield of ester. The reasons for the necessity of water are unclear, however, it may be due to creation of a proton transfer network within the IL. 

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The field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

In 1999 Blanchard et al. reported a good solubility of carbon dioxide in l-butyl-3-methylimidazolium hexafluorophosphate at high pressures, while the ionic liquid did not dissolve in carbon dioxide. Therefore, supercritical carbon dioxide is suited to extract organic solutes from ionic liquids, and also continuous flow homogeneous catalysis in ionic liquids carbon dioxide systems is possible. First spectroscopic studies show that the anion dominates the interactions with carbon dioxide by Lewis acid-base interactions. However, the strength of carbon dioxide anion interactions did not correlate with carbon dioxide solubility. Thus, strong anion-carbon dioxide interactions were excluded as major cause for the carbon dioxide solubility in ionic liquids. Instead, a correlation of carbon dioxide solubility and the ionic liquid molar volume was observed. Additionally, a significant volume decrease of dissolved carbon dioxide was... 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

Another example is butene dimerization catalyzed by nickel complexes in acidic chloroaluminates 14). This reaction has been performed on a continuous basis on the pilot scale by IFF (Difasol process). Relative to the industrial process involving homogeneous catalysis (Dimersol process), the overall yield in dimers is increased. Similarly, selective hydrogenation of diene can be performed in ionic liquids, because the solubility of dienes is higher than that of monoene, which is higher than that of paraffins. In the case of the Difasol process, a reduction of the volume of the reaction section by a factor of up to 40 can be achieved. This new Difasol technology enables lower dimer (e.g., octenes) production costs 14). 

Some ionic liquids have tunable Lewis acidities and basicities. The tuning can be achieved simply by varying the anion fraction in the overall ionic liquid composition. In some cases, Bronsted acidity can also be introduced into stable ionic liquids. Many publications show the broad applicability of acidic or basic ionic liquid media in catalysis replacing corrosive liquids and solid catalysts. 

Several authors reported the use of ionic liquids containing protonic acid in catalysis (118-120). For example, strong Bronsted acidity in ionic liquids has been reported to successfully catalyze tetrahydropyranylation of alcohols (120). Tetra-hydropyranylation is one of the most widely used processes for the protection of alcohols and phenols in multi-step syntheses. Although the control experiments with the ionic liquids showed negligible activity in the absence of the added acids, high yields of product were obtained with the ionic liquid catalysts TPPTS or TPP.HBr-[BMIM]PF6. By rapid extraction of the product from the acidic ionic liquid phase by diethyl ether, the reaction medium was successfully reused for 22 cycles without an appreciable activity loss. A gradual loss of the catalyst and a reduced volume of the ionic liquid were noted, however, as a consequence of transfer to the extraction solvent. 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

It is also relatively easy to functionalise imidazolium cations at the 2-position.[88] For example, the phosphine derivatised salts shown in Figure 2.7 illustrate such a substitution pattern and they are easily prepare by virtue of the acidity of the 2-proton.[74] Substitution of the 2-proton tends to yield relatively high melting salts instead of ionic liquids. Despite this limitation the imidazolium-phosphine compounds are good ligands for catalysis improving the immobilisation potential of complexes dissolved in ionic liquids. 

From conventional homogeneous to green homogeneous and heterogeneous catalysis with Lewis acids (ring opening and electrocyclic formation of heterocycles, reactions in ionic liquids, 7V,7V -dialkylimidazolium and Y-alkylpyridinium salts) 03CRV4307. 

Lewis acids immobilized on ionic liquids have been used as the acid catalysts for the alkylation of phenols. The catalytic activities of the immobilized ionic liquids were found to be higher than those for the zeolites. Typically, ionic liquids such as butylmethyUmi-dazoUum halides are treated with AICI3 to give the ionic liquids with halogenoaluminates as the counter anions. They show enhanced Lewis acid character and promote predominantly C-alkylation of phenols over O-alkylation. The alkylation of phenol with dodecene, for example, in the presence of these immobilized ionic liquids results in up to 70% of C-alkylated products (ortho and para products) and 30% of O-alkylated product, comparable to zeolite catalysis (equation 23). The rates of alkylation of phenols are slower than those of arenes due to the complexation of the phenolic group with the Lewis acidic ionic liquids. At higher temperatures conversions of up to 99% could be achieved. 

In another approach, the dehydration has been conducted in ionic liquids (l-alkyl-3-methylimidazolium chloride) under acid or metal chloride catalysis yielding HMF up to more than 80% nearly without any formation of by-products like levulinic acid [32], When using chromium(II) chloride as catalyst even D-glucose could be used as feedstock since this catalyst is effective for the in situ isomerization of o-glucose to o-fructose before dehydration takes place to produce HMF in 70% yield. The catalyst system N-heterocyclic carbene/CrCU in l-butyl-3-methyl imidazolium chloride has been developed for the selective conversion of D-fructose (96% yield) and o-glucose (81% yield) [33]. 

The regioselective hydroformylation of functionalized and nonfunctionalized olefins can also be performed by platinum compounds [26] in chlorostannate ionic liquids as solvents for homogeneous catalysis (entries 20-22, Table 6.1). Dissolved in chlorostannate ionic liquids, the Pt catalyst shows enhanced stability and selectivity in the hydroformylation of methyl-3-pentenoate compared to the identical reaction in conventional organic solvents. The moderate Lewis acidity of these ionic liquids allows the activation of the Pt catalyst combined with tolerance of the functional groups in the substrate. In the case of 1-octene hydroformylation, a biphasic reaction system could be performed using the chlorostannate ionic liquid. 

In 2012, Ramos and collaborators also studied this mechanistic pathway by means of ESI-MS, NMR, and theoretical calculations in their approach [9]. In this work, the mechanism of the Biginelli reaction was explored by studying the influence of Lewis acid catalysts in the presence of ionic liquids. Once again, the researchers noticed the exclusive formation of iminium intermediate m/z 149 (Figure 4), indicating that xmder Lewis Acid catalysis and in ionic liquids, the preferred mechanistic pathway was the iminium mechanism [9]. 

Baum, S., van Rantwijk, E, and Stolz, A. (2012). Application of a recombinant Escherichia coli whole-cell catalyst expressing hydroxynitrUe lyase and nitrilase activities in ionic liquids for the production of (S)-mandelic acid and (S)-mandeloamide. Advanced Synthesis Catalysis, 354,113-122. 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

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